The present invention relates in general to the art of petroleum recovery and processing, and in particular, to a composition and process for its use to dissolve sulfur scale and render passages open for gas and oil recovery.
Subterranean formations containing oil and natural gas (or either) are susceptible to treatment to improve the recovery rate and total amount recovered of these values. Increasing the permeability of a formation containing the sought after value can increase the ability of the value to flow to a well. Enhanced permeability may release values from a formation itself, or improve or obtain a flow path to pockets and the like of the value. Thus, improving existing channeling and fissuring of a formation can improve recovery. Obviously, the creation of new fissures and channels has the same effect. Increasing the effective diameter of a well in the zone of a formation containing the value also increases recovery by reducing the resistance of flow to a well. Formation treatment to increase recovery both in amount and rate has included steam pushing, hydraulic, explosive, acid treatment, and combinations of these.
Sulfur scale quite often builds up in the interstices of formations serving as petroleum passages. The build-up can block petroleum flow in time. Sulfur scale is a broad term covering many different sulfur-containing substances.
While many formations are amenable to relatively easy treatment to increase natural gas and oil value recovery and production rates, when they become bound by sulfur scale, treatment has often proved impossible.
Sulfur scale appears in a wide variety of formations. One formation that it appears in is dolomite. Dolomite is a heterogeneous, variable composition of magnesium carbonate (MgCO.sub.3), a calcium carbonate (CaCO.sub.3), calcium magnesium carbonate, CaMg(CO.sub.3).sub.2, a complex of magnesium carbonate and calcium carbonate, and other constituents such as magnesium oxide, silicates, complex aluminum oxides, iron oxides, and various metallic salts.
A known method of improving carbonaceous recovery and production rate in dolomite has been by hydrochloric acid (HCl) with salts of copper or arsenic as inhibitors. While hydrochloric acid dissolves a good percentage of the carbonates of the dolomite, a significant undissolved quantity of carbonate remains despite the fact that each carbonate by itself is essentially completely soluble in the acid. Moreover, at least the silicates of dolomite are not effectively dissolved by the acid. Hydrochloric acid intensified with hydrofluoric acid (HF) does not significantly improve dissolution over hydrochloric acid alone. The use of such acids as sulfuric acid on dolomite have not proved effective either. Sulfuric acid dissolves the magnesium carbonate constituent but produces the insoluble salt calcium sulfate as a precipitate, which again inhibits the treatment, often to the point of rendering the treatment of little or no benefit. It is now through that sulfur scale shields dolomite from attack and that this shielding largely accounts for the undissolved fraction of dolomite in the hydrochloric acid system.
It is not only in the improvement of permeability in the formations for the flow of oil and gas which presents a problem, but it is also in drilling. For example, dolomite is a tough material and it is difficult to drill through; it might appear in zones above the petroleum bearing zones and must be penetrated during the drilling process.
It is known that corrosion inhibitors may be required to protect drill string and well casing during acid treatment.